In some circumstances, the female threaded area of a bolt hole suffers from a variety of service induced failures, which weakens the strength of the mechanical joint associated with the bolt hole. Material failure may occur in power plant equipment, such as steam turbines in the area of the steam chest, where high temperatures induce failure in the threaded areas of the bolt holes in valves made of carbon steel or low alloy chromium-molybdenum steels. Examples of steam turbine apparatus which are known to have experienced service induced failures include the bolt holes in throttle valves, governor valves, reheat stop valves, and interceptor valves. The type of failure which occurs in the mechanical joints of these valves includes creep damage, fretting corrosion, overload, and mechanically or thermally induced fatigue.
Currently known methods for replacing failed material in the female threaded area of a bolt hole require mechanical removal of the failed material from the internal diameter of the bolt hole, thus leaving a new hole in metallurgically acceptable material, but with a larger internal diameter than the preexisting hole. Subsequently, an insert is placed into the new hole. Inserts which are commonly known in the art include helicoils or mushroom studs. The insert brings the hole to its originally specified internal diameter after the insert is threaded, but does not account for the structural strength lost due to enlargement of the hole diameter and concomitant reduction in the material proximate the hole.
In turbine apparatus such as the valves discussed above, wherein a series of bolt holes are circumferentially located and spaced apart on the periphery of the mechanical joint, removal of the failed material results in removal of material from the ligaments of the joint between the bolt holes. A problem has been recognized in that the removal of material from the ligaments of the joint greatly reduces the structural strength of the joint. Placement of an insert into the bolt hole, to allow mechanical fastening by returning the hole to its original internal diameter, does not, however, return the ligament to its original strength. Thus, the overall structural strength of the joint must be sacrificed significantly in order to repair the failed material in the bolt hole. This problem is even more compounded where a plurality of adjacent bolt holes in a particular valve joint must be repaired.
Alternative methods for repairing a failed bolt hole, from a valve for example, include removing a section of the valve which encompasses the bolt hole, followed by welding an entire new section to the remaining structure and then drilling a new bolt hole. This method has been found to be undesirable as the major fabrication effort requires complete post weld heat treatment which can cause unacceptable distortion and in some cases weakens the strength of the associated joint. Another method comprises replacing the entire valve joint. The cost and complexity associated with such a method is obvious.
Therefore, there is a need for a method for replacing the failed material in the female threaded area of a bolt hole which not only retains the original structural integrity, but also provides for metallurgical enhancement of the joint in the area of the failed material. The present invention provides a method which satisfies this need.